Second-graders from P.S. 51  the Elias Howe School, at 520 W. 45th St.  visited UPSs main Manhattan facility at W. 43rd St. and 12th Ave. last month. David Goldstein, UPS community relations manager, in photo at right, showed the children how the package conveyor belt and a hand-held delivery-recording device work. At left, a student displayed her UPS I.D. PENCIL, the Principal for a Day programs sponsor, coordinated the event.